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Publications / Blogs

Space heating and energy system flexibility

This blog was provided by Seamus Garvey, Professor of Dynamics at the University of Nottingham.

Seamus Garvey is a Professor of Dynamics at the University of Nottingham. His research focuses on ways to restore flexibility in a future Net Zero electricity grid in the UK.

Heating our homes and workplaces presently accounts for a large fraction of all man-made CO2 emissions in the UK. Everyone agrees that the thermal insulation of homes should improve. This alone will not remove the requirement for some heating during cold winter periods and we must stop burning natural gas. The two strategies commonly discussed are (1) electrically-driven heat pumps and (2) burning hydrogen circulated in (parts of) the existing gas network – a network already connected to 20 million properties.

At first sight, electric heat pumps appear obviously preferable. The “coefficient of performance” (CoP) of a heat pump describes the ratio between heat energy pumped into a space and electrical energy consumed. For good heat pumps the CoP may average around 4. Producing hydrogen from renewable energy via electrolysis and then burning that for heating delivers an effective CoP below 0.7.

Things are not so simple for three main reasons. Firstly, the energy consumed for heating is huge; in the UK, annual heat energy consumption presently exceeds electrical energy consumption by >2.5. Secondly, the ratio between peak and average consumption is far greater for gas than for electricity – and the seasonal variation is also much stronger. Finally, we must take seriously that wind power will provide at least 70% of generated electricity in Net Zero UK and the intermittency of cumulative UK wind power operates on timescales of 10s-1000s of hours. These three separate points support one main truth: how we will manage space heating in the future will be critical to electricity grid operation.

Integrating some thermal storage into (or alongside) properties is one major opportunity for cost-effective action that deserves far more attention. Incorporating phase-change materials into water tanks and into the fabric of buildings is one superb way to do this. Including much more hot water storage is also very attractive – especially if done at construction stage and for community developments. Insulating homes on the outside is another very effective measure. This allows the main structure of the property to serve also as thermal store. Energy storage in the form of heat is at least two orders of magnitude less expensive than using batteries for the same amount of energy and is suitable for retaining heat for ~10hrs. This distributed energy storage can transform the way that electricity distribution operates by “keeping the wires full” for much more of the time – using heat pumps mainly when other loads are low.

A research project called “GasNetNew” presently at the Universities of Nottingham, Loughborough, Birmingham and Portsmouth is examining various ways in which the existing gas network might serve when gas boilers can no longer be used. One idea being examined involves distributing non-potable water in the network which can be used as an effective source of low-grade heat when air temperatures are below 0°C. Then, the CoP for heat pumps need not fall sharply at the very times when it is most important. The non-potable water is turned into an ice slurry and discarded into drains where it melts again.

The intermittency characteristics of wind indirectly strengthens the case for using hydrogen for at least some heating. Although other energy storage is better for shorter timescales, hydrogen stored in salt caverns provides very low-cost energy storage suited for variations on timescales of many hundreds of hours. Even if that hydrogen is burned directly in specially-adapted boilers in some properties, the fact that we can avoid losing 50% of the available energy, and a lot of hardware cost, by not converting the hydrogen back to electricity in fuel-cells or engines is significant compensation for the “poor CoP”.

One alternative to simple hydrogen combustion for heating is to run hydrogen-powered “CHP” (combined heat and power) plant. Such a plant would be viable at the scale of multiple MW and would mainly suit new-build developments comprising many tens of properties that might directly utilise the low-grade heat produced.

A second alternative is being explored in a UKRI-funded research fellowship presently held at the University of Nottingham. This involves combining a hydrogen engine and a heat pump into a single relatively-simple machine whose primary purpose is to deliver ~2.5 times more heat into a property than the calorific value of the hydrogen. With clever design, such a machine might have only one moving part and could even be configured to produce a small amount of electricity in use. Then, at times of very cold weather some properties might be receiving, say, 8kW of heat whilst simultaneously generating 500W of electricity to offset high electricity demand present elsewhere from heat pumping.

Future space heating needs some systems thinking far in advance.